Based on the Brandon Lee case, if they were using squibs (powder removed) rounds during the earlier days, they very likely could've gotten a bullet stuck in the barrel. The squibs would be used to film scenes that show the ammunition, so it looks normal. They should have the primers removed to avoid that.

After getting a bullet stuck in the barrel, if they used blanks the following day, it could clear most of the safety checks and still launch that sucker with nearly the full muzzle energy of proper ammunition. And being we don't know what is in a blank round (likely faster burning powder, likely over-loaded) it M.E. may be unpredictable and might even exceed factory pressures/specs.

In any event, it sounds like that is a likely possibility instead of "someone didn't notice it had bullets". It also very closely mirrors the Brandon Lee case.